Method for adjusting an actuator

ABSTRACT

An actuator which is displaced by a force in one end position and is displaced into the other end position by an adjusting device that is controlled using a pulse duration modulated signal. If a quasi-stationary condition is identified, in which despite repeated control intervention the actual position of the actuator lies outside a targeted range around the desired position, the retaining pulse duty factor which is used by the adjusting device to main the actuator in one position is modified based on the distance from the desired position. If drift is identified, the drift behavior is determined and the retaining pulse duty factor is modified according to said drift behavior.

This application is made pursuant to 35 U.S.C. §371 of internationalapplication number PCT/DE01/01222, filed Mar. 30, 2001 with a prioritydate of Apr. 14, 2000.

FIELD OF THE INVENTION

The invention relates a method for adjusting an actuator which can movebetween two end positions, which is displaced into one end position andwhich can be moved to the other end position by means of an adjustingunit.

BACKGROUND OF THE INVENTION

Actuators of the type in question here, which are displaced into one endposition and can be moved into the other end position by means of anadjusting unit, must therefore be held in a desired position by activelyactivating the adjusting unit. From a held position, it is possibleeither to bring about adjustment into the one end position by suspendingthe activation of the adjusting unit, or to bring about adjustment intothe other end position by increased activation of the adjusting unit. Aconvenient way of activating such an adjusting unit, which may, forexample, operate electromagnetically, is actuation with apulse-width-modulated signal. Depending on the pulse duty factor of thepulse-width-modulation, adjustment is carried out into one end positionor the other end position. If the actuator is to be held in oneposition, the adjusting unit must be actuated with a retaining pulseduty factor.

The actuators which are described are preferably used in devices forcamshaft phase adjustment in internal combustion engines. Such acamshaft phase adjuster is described, for example, in DE 43 40 614 C2.It is a typical example of an actuator which is influenced by anadjusting unit and in which dead times and delayed response requirelimitation of the maximum achievable adjustment speed and consequentlycorresponding parametrization of the associated adjuster.

Owing to these dead, times and the delayed response, it is not possibleto equip the adjuster which adjusts the actual position of the actuatorwith an integral component as otherwise an unstable system would beproduced. Instead, a certain maximum control error, below which theadjuster is not active, is permitted.

However, this procedure leads in such cases to difficulties in which theactual position of the actuator cannot be measured continuously butrather only sampling is possible. There are then cases in which, despiterepeated adjusting intervention, the desired position is not reached butthere is instead a quasi-steady or drifting state of the actuator inwhich the actuator exhibits a constant control error or a continuousmovement to an end position.

Accordingly, there is a need, for a method of adjusting an actuator ofthe type described, with which precise adjustment to a desired positioncan be reached without quasi-steady or drifting states occurring.

Other needs will become apparent upon a further reading of the followingdetailed description taken in conjunction with the drawings.

SUMMARY OF THE INVENTION

The invention is based on the idea that the retaining pulse duty factoris, of course, the same for all the operating states of the actuatoronly in the rarest of cases. Although an actuator can be configured insuch a way that the retaining pulse duty factor is the same for all theactual positions of the actuator, this cannot be achieved for alloperating conditions, for example temperatures, supply voltages,hydraulic pressures or the like. If the retaining pulse duty factor doesnot have precisely the value which is necessary to keep the actuator inan actual position, it will move toward an end position. A faultyretaining pulse duty factor is thus the cause of a quasi-steady ordrifting state. A quasi-steady state is found if, despite repeatedadjusting intervention, a minimum control error is continuouslyexceeded. The retaining pulse duty factor is then changed until thecontrol error drops below a threshold value.

In the case of the drifting state of the actuator, the drift behavior isdetermined and the retaining pulse duty factor is correspondinglycorrected until the desired position is maintained precisely within adesired framework.

The difference between a quasi-steady and drifting state is caused bythe fault in the retaining pulse duty factor. When there is a relativelylarge fault in the retaining pulse duty factor, a quasi-steady statewill be established. Between the times of the sampling measurement ofthe actual position, the actuator drifts out of the acceptable controlerror so quickly that a constant control error is measured despiterepeated control interventions. On the other hand, in the driftingstate, the fault of the retaining pulse duty factor becomes relativelysmall. Here, the movement of the actuator out of the desired positiontakes place so slowly that one or more measurements exhibit an actualposition within the acceptable control error. This makes it possible todetermine the drift behavior, and calculate precisely the necessarycorrection of the retaining pulse duty factor from it.

As the retaining pulse duty factor may need to be corrected not only asa result of operating states of the actuator, but it may also need to bechanged due to a defect in the actuator, a defect in the actuator isdetected if the change in the retaining pulse duty factor appearsnecessary beyond a specific pulse width modulation. The actuator is alsodefective if correction of the retaining pulse duty factor is repeatedlynecessary over a time period, that is to say no fixed retaining pulseduty factor can be found during the control over a relatively long timeperiod during which the acceptable control error is maintained.

Owing to the dead times and the delayed response behavior of theactuator, it would of course be desirable to configure the adjuster tobe as immune to oscillation as possible. On the other hand, in manyapplications, for example in the aforementioned camshaft phaseadjusters, rapid re-adjustment into a new desired position is required.These, in themselves, contradictory objectives can be achieved in onepreferred development by virtue of the fact that large jumps in thedesired position can be achieved by pilot control and the adjuster isactive only in a narrow range around the respective desired position.Here, the adjuster can be permitted only a certain maximum change of thepulse width modulation, which has positive effects on the stability.This maximum change is preferably dependent on the adjustment to bebrought about in the actual position, which leads to the actual positionbeing adjusted in a non-oscillating way to the, desired position, evenwhen there are relatively long dead times.

In an actuator which is embodied as a camshaft phase adjuster, thesampling of the position of the camshaft, and thus the determination ofthe position of the actuator, generally takes place once or twice perrevolution of the camshafts, in that a semicircular disk which isattached to the camshaft is sensed. The selection of the retaining pulseduty factor can be given a two-stage configuration for such a camshaftphase adjuster. On the one hand, a basic value for the retaining pulseduty factor is obtained from a basic characteristic diagram which takesinto account operating parameters of the internal combustion engine, forexample operating temperature, oil pressure, battery voltage or thelike. On the other hand, the aforementioned correction of the retainingpulse duty factor can be obtained from an adaptation characteristicdiagram which covers the constant control error or one or moreparameters which characterize the drift behavior. Advantageousrefinements of the invention are the subject matter of the subclaims.

These and other features and advantages of the invention will beapparent upon consideration of the following detailed description of thepreferred embodiment of the invention, taken in conjunction with theappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of an internal combustion enginewith camshaft phase adjustment,

FIG. 2 shows a camshaft with a cut-open mechanical adjusting part,

FIG. 3 shows a block circuit diagram of the control circuit for camshaftphase adjustment,

FIG. 4 shows the relationship between the pulse width modulation factorand adjustment speed of the actuator,

FIG. 5 shows, the variation range of the pulse width modulation which isaccessible to the adjuster, as a function of the control error, and

FIGS. 6 show time sequences of the actual position of to 8. theactuator.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is capable of embodiment in various forms,there is shown in the drawings and will be hereinafter described apresently preferred embodiment with the understanding that the presentdisclosure is to be considered as an exemplification of the invention,and is not intended to limit the invention to the specific embodimentdescribed and illustrated.

In the figures, elements with identical design and function are providedwith the same reference symbols.

An internal combustion engine which is shown schematically in FIG. 1comprises a cylinder 1 with a piston 11 and a connecting rod 12. In theschematic drawing in FIG. 1, only one cylinder is illustrated, but ofcourse an internal combustion engine is generally a multicylinderinternal combustion engine. The connecting rod 12 is connected to apiston 11 and a crankshaft 2. A first gearwheel 21 is seated on thecrankshaft 2 and is coupled via a chain 21 a to a second gearwheel 31which drives a camshaft 3. The camshaft 3 has cams 32, 33 which activatethe charge cycle valves 41, 42.

In order to adjust the position or phase of the camshaft 3 in comparisonwith the crankshaft 2, an actuator, 5 is provided. It has a mechanicaladjusting part 51 which is supplied by an electromagnetically activatedtwo/three-way valve 54 via hydraulic lines 52, 53. The valve 54 isconnected to an oil reservoir via a high pressure hydraulic line 55 anda low pressure hydraulic line 56, and an oil pump (not illustrated)generates the pressure in the high pressure hydraulic line 55.

A control unit 6 actuates the valve 54 by means of an actuation signalTVAN_S. The control unit 6 predefines the actuation signal TVAN_S hereas a function of the values of various sensors 71 to 74. These aresensors for measuring the rotation speed N, the crankshaft angle of thecrankshaft 2, the camshaft position NWIST, the air mass MAF sucked in bythe internal combustion engine and the temperature TOEL of the oil whichdrives the adjusting part 51. Of course, this sensor equipment is to beunderstood only by way of example.

FIG. 2 shows the camshaft 3 with the mechanical adjusting part 51 as apartial sectional view. The mechanical adjusting part 51 is driven bythe second gearwheel 31 in which a third gearwheel 511 is seated in apositively locking fashion. This third gearwheel 511 has an internalbeveled toothing which engages in an assigned external beveled toothingof a crown gear 512 which is seated in the third gearwheel 511. Thiscrown gear has a drilled hole with straight toothing which engages in acorresponding toothing of a fourth gearwheel 513. This ensures that,irrespective of the axial position of the gearwheel 512, the fourthgearwheel 513 which is mounted on the camshaft 3 does not change itsaxial position, although the crown gear 512 is connected fixed in termsof rotation to the fourth gearwheel 513.

Depending on the oil pressure in the hydraulic lines 52, 53, the crowngear 512 is then displaced axially. with respect to the camshaft.Brought about by the engagement of the external beveled toothing of the,crown gear 512 and of the internal beveled toothing of the thirdgearwheel 511 one in the other, the camshaft 3 rotates with respect tothe third gearwheel 511 which is connected fixed in terms of rotation tothe second gearwheel 31.

A spring 514 displaces the crown gear 512 away from the. camshaft 3, andthus adjusts the phase of the camshaft 3 toward an end position. Bymeans of the oil pressure in the hydraulic lines 52, 53 it is possibleto bring about an adjustment, indicated schematically by dashed lines inFIG. 2, of the phase of the cam 32 with respect to the second gearwheel31 which drives the camshaft 3.

The actuating device 5 thus brings about a phase adjustment of thecamshaft 3 in relation to the crankshaft 2. The phase can be adjustedcontinuously within a predefined range. If both the camshaft 3, which isused to activate the inlet charge cycle valves, and a camshaft foractivating the outlet charge cycle valves are correspondingly providedwith an actuator 5, it is possible to vary the start of the stroke andthe end of the stroke of the charge cycle valves which are predefined bymeans of the shape of the cam.

The method of operation of the valve 54 is relevant to understanding theinvention only insofar as the energization of the electromagnet 57 setsthe pressure electromagnet 57 is not energized, no pressure acts on thecrown gear 512, for which reason there is no force opposing the spring514, and the crown gear 512 is moved into its axial end position, awayfrom the camshaft 3. This corresponds to an end position of the camshaftphase adjustment range. If the electromagnet 57 is energized to amaximum extent, the other end position of the camshaft phase adjustingrange is reached. For the purpose of energization, the electromagnet 57is actuated with the actuation signal TVAN_S in a pulse-width-modulatedfashion.

In order to hold the actuator 5 in a specific position, the actuationsignal TVAN_S is pulse-width-modulated with a retaining pulse dutyfactor. The retaining pulse duty factor is selected here in such a waythat the pressure in the hydraulic line 52 which acts on the crown gear512 precisely compensates the force, of the spring 514 in a desiredposition of the crown gear 512. The spring 514 is configured in such away that the force exerted by it is identical for each position of thecrown gear 512. The retaining pulse duty factor is then the same for allthe camshaft phase positions. The retaining pulse duty factor is, forexample, in the vicinity of 50%. Of course, the retaining pulse dutyfactor can also depend on the camshaft phase adjustment, but this is notassumed in what follows.

In order to move the camshaft, phase adjustment means from one specificposition to the other, when there is an adjustment which signifies anincrease in pressure, the electromagnet 57 is energized to a greaterextent. Although, depending on the design, a greater degree ofenergization would also result in a reduction in the pressure in thehydraulic line 52, it is assumed in what follows that a greater degreeof energization of the electromagnet 57 brings about an increase in thepressure in the line 52.

FIG. 3 shows, as a block circuit diagram, the control circuit forcamshaft phase adjustment. The control unit 6 has an adjuster 61. Itcontinues to measure the position of the camshaft 3 by means of thesensor 72 by sensing a semicircular disk which is mounted on the secondgearwheel 31. The signal NWIST of the sensor 73 is converted, in thecontrol unit 6, into an actual position I of the actuator 5 asultimately only the latter is of interest for the adjuster 61. Theadjuster 61 outputs the actuation signal TVAN_S to the solenoid valve54. The actuation signal is pulse-width modulated with a factor P. Thesolenoid valve 54 brings about an adjustment of the actuator 5 counterto the force of the spring 514.

As the solenoid valve 54 controls the hydraulic flow to the mechanicaladjusting part 51, the adjusting speed which is brought about here isnot linearly dependent on the factor P of the pulse-width modulation.The relationship is plotted in FIG. 4. Given a pulse-width modulationfactor P of zero, a maximum adjusting speed v of 100% is reached, and inthis case the adjustment is carried out exclusively by means of thespring 514. When there is a maximum pulse width modulation factor P of100%, i.e. when there is continuous energization of the solenoid valve54, the adjustment to the other end position takes place at a maximumspeed v. When there is a factor P of the pulse width modulation of h,the actuator 5 is held, for which reason this factor h is referred to asretaining pulse duty factor. Small deviations in the retaining pulseduty factor h lead to a relatively small adjusting speed. The shape ofthe curve in FIG. 4 makes it possible to configure the adjuster 61 to bestable by allowing it only a restricted range of the factor P of thepulse width modulation around the pulse duty factor h. This isrepresented in FIG. 5 in which the variation dP of the factor P which ispermitted to the adjuster is plotted as a function of the control errord which results from the difference in absolute value between thedesired position S and actual position I. When there is a control errord=0, the variation dP allowed to the adjuster is 5%. As the controlerror increases, it rises to a maximum value of, for example, 15%. Thisconfiguration of the adjuster 61 brings about a stable control behavior.In order, nevertheless, to be able to ensure a high adjusting speed,when there are large jumps in the desired position S, the adjuster 61 issupported by the control unit 6 by means of a prior control. For thispurpose, the control unit 6 changes the factor P of the pulse widthmodulation of the actuation signal TVAN_S by a certain degree for acertain time period until the desired position jump to be carried out isachieved to a certain degree, for example 80%.

The remaining change in the desired position is then left to theadjuster 61, which reaches the new desired position, without oscillationon the basis of the configuration illustrated in FIG. 5.

In order to configure the adjuster 61 so as to be stable, in addition tothe limitation of the variation dP which is described in FIG. 5, thereis provision for the adjuster 61 to perform an adjusting interventiononly when there are certain minimum control errors dmin, on whichdetails will be given later.

The retaining pulse duty factor h must, as mentioned above, be selectedsuch that the actuator 5 holds its actual position. For this purpose,the force of the spring 514 must be compensated by the pressure in thehydraulic line 52. In the case of an actuator 5 which is not displacedinto the one end position by a spring 514 but rather by the activationforces of the cams 32, 33, these forces must be compensated.

The retaining pulse duty factor h depends on various operatingvariables. These are, on the one hand, the temperature and the pressureof the hydraulic fluid in the hydraulic lines 52, 53, 55 and 56. On theother hand, the battery voltage during the energization of theelectromagnet 57 has an effect. The retaining pulse duty factor h isthus taken from a characteristic diagram as a function of theseoperating parameters. With the solenoid valve 54 described here it isapproximately 50%. In contrast, when activation is not hydraulic butrather purely electromagnetic, it will differ greatly from this, beingfor example 4%.

When the retaining pulse duty factor h has been obtained from thecharacteristic diagram, it is still possible for a permanent controlerror d to be established, as is shown in FIG. 6. FIG. 6 shows theactual position I of the actuator, and thus of the camshaft phase, as atime sequence. The dashed line shows the desired position S. Thedot-dashed line shows the acceptable control error, and curve 8illustrates the actual position I of the actuator 5 which is sampled atthe measurement point 10. As the sampling frequency depends on therotational speed of the camshaft owing to the sampling of thesemicircular wheel on the second gearwheel 31, the measurement points 10in the case illustrated are too far apart from one another to representthe actual profile of the curve 8. Undersampling occurs, which does notfulfil the sampling theorem. As a result, the curve 9 which isillustrated with dashed lines appears as a virtual position of theactuator 5. The minimum control error, below which adjustingintervention must not be performed for reasons of stability, is enteredas dmin.

In the case illustrated, the retaining pulse duty factor h is incorrect,for which reason the actuator 5 moves out of the desired position. Atthe time t₀ it will be assumed, for the sake of illustration, that theactual position I is the same as the desired position S. Owing to theincorrect retaining pulse duty factor h, the actuator moves out of thedesired position S. It is only during the second measurement of theactual position at the time t₁ that the adjuster 61 determines that anadjusting intervention is necessary as the minimum control error dminhas been exceeded. For the adjusting intervention, the solenoid valve 54is briefly energized with a factor P of the pulse width modulation whichdiffers from the retaining pulse duty factor h. Although the actuator 5is moved into the region of the acceptable control error, here even thedesired position S, the acceptable control error has already beenexceeded again by the next measurement point. It is only at thesubsequent measurement point, at the time t₂, that the adjuster 61 hasan opportunity for an adjusting intervention as it is only then that theminimum control error dmin is exceeded. The position of the measurementpoints 10 therefore results in beats in a quasi-steady state in whichnone of the measurement points 10 lies within the acceptable controlerror around the desired position S. The system does not leave thisquasi-steady state outside the acceptable control error, although theadjuster performs adjusting interventions at the times t₁, t₂, t₃, t₄,etc., as the error of the retaining pulse duty factor h is so largethat, by the next measurement, the actual position I already deviatessignificantly from the desired position S again, and the acceptablecontrol error is exceeded.

In order to avoid or leave this quasi-steady state, the retaining pulseduty factor h is then changed if the control unit 6 detects that,despite an adjuster intervention at the time t₁, the next measurementpoint lies outside the acceptable control error. This is illustrated inFIG. 7. Up to the first measurement point after the time t₁, the timesequence in FIG. 7 does not differ from the time sequence in FIG. 6. Ifthe control unit determines, with the first measurement point after thecontrol intervention at the time t₁, that the actual position I liesoutside the acceptable control error S, the retaining pulse duty factorh is changed at the time t_(e1), in this case reduced. The reduction inthe retaining pulse duty factor h leads to a decrease in the drift withwhich the actual position I moves away from the desired position S.However, at the time t₂ the minimum control error dmin is exceeded,which leads to renewed adjusting intervention. Then, the retaining pulseduty factor h can be changed again with a further correction, as aresult of which the actual position I moves away from the desiredposition S even more slowly. This further correction of the retainingpulse duty factor h takes place at the time t_(e2) at which it becomesapparent that the acceptable control error is exceeded again. This isnot the case with the first measurement after the time t₂, owing to thecorrection at the time t_(e1) at which the retaining pulse duty factor hhas already been more, satisfactorily approximated to the actual value,but rather only with the second measurement. Only after this measurementis a correction of the retaining pulse duty factor performed at the timet_(e2).

With this second correction at the time t_(e2), the error of theretaining pulse duty factor is so small that the drift of the retainingpulse duty factor is slowly toward the sampling rate of the measurementof the sensor 72 which leads to spacing apart of the measurement points10. After an adjusting intervention which occurs whenever the minimumcontrol error dmin is exceeded, there are always a number of measurementpoints 10 which lie within the acceptable control error. A quasi-steadystate outside-this control error therefore no longer occurs.

This state in which a slow drift is determined is illustrated in FIG. 8.It is then possible to determine the drift speed or the drift behaviorof the actual position I precisely as a plurality of measurement points10 lie within the acceptable control error. The curve 8 of FIG. 10 canbe conceived of as a continuation of the curve 8 in FIG. 7 if it isconsidered starting from the time t_(e2). The drift state of the actualposition I illustrated in FIG. 8 can, however, also be presentindependently of the previous state of FIG. 7. It always occurs if theretaining pulse duty factor h is, relatively close to the target valuebut is nevertheless incorrectly too large or too small. As in this driftcase, the measurement points 10 are close enough to one another tofulfil the sampling theorem approximately, the drift behavior can bedetermined from the position of the measurement points 10 and acorrection of the retaining pulse duty factor can be determined directlytherefrom as follows:

D=[I(t ₃)−I(t _(e2))]/[(t ₃ −t _(e2))·I(t ₃)].

Here, I(t) is the actual position at the time t,t_(e2) is the time atwhich the acceptable control error is exceeded, and t₃ is the time atwhich dmin is exceeded. The drift factor D which is given by thisequation can be used directly from multiplicative correction of theretaining pulse duty factor h. It expresses the percentage increase inthe drift illustrated in FIG. 8. It permits fine correction of theretaining pulse duty factor h in the cases in which the drift can bedetermined, i.e. if the drift is slow toward the sampling rate of themeasurements of the sensor 72. The correction of the retaining pulseduty factor h which has been described with reference to FIGS. 6 and 8can also be achieved by accessing a characteristic diagram in which thecorrection of the retaining pulse duty factor h is stored as a functionof the error in the quasi-steady state in FIG. 6, or the drift behaviorin the case of FIG. 8. This characteristic diagram makes it possible todispense with the calculation of the drift factor D in the equationdesignated above. A time period can, for example, be input into thischaracteristic diagram. This may be the time period which passes betweenthe start of the retaining mode with the retaining pulse duty factor hand the first time the minimum error dmin is reached or exceeded. Thecorresponding correction factor for the retaining pulse duty factor hcan then be determined from this time by means of the characteristicdiagram.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description, and is notintended to be exhaustive or to limit the invention to the precise formdisclosed. The description was selected to best explain the principlesof the invention and their practical application to enable othersskilled in the art to best utilize the invention in various embodimentsand various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention not belimited by the specification, but be defined by the claims set forthbelow.

What is claimed is:
 1. A method for adjusting an actuator which can movebetween two end positions, which is displaced into one end position andcan be moved into the other end position by activating an adjustingunit, the method comprising: sensing the actual position of theactuator; adjusting the actuator to a desired position using apulse-width-modulated actuation of the adjusting unit; holding theactuator in the desired position by actuation using a retaining pulseduty factor and, only if a minimum adjustment of the actuator isnecessary, an adjusting intervention using the retaining pulse dutyfactor of a deviating pulse width modulation; and correcting theretaining pulse duty factor if, despite repeated adjusting intervention,a deviation between the desired position and actual position iscontinuously measured, until the deviation drops below a thresholdvalue.
 2. The method as claimed in claim 1, wherein a defect in theactuator is detected if the correction of the retaining pulse dutyfactor is necessary beyond a specific pulse width modulation.
 3. Themethod as claimed in claim 1, wherein a defect in the actuator isdetected if the correction of the retaining pulse duty factor isnecessary beyond a predetermined time period.
 4. The method as claimedin claim 1, wherein a control intervention brings about only ascertainmaximum change in the pulse width modulation.
 5. The method as claimedin claim 4, wherein the maximum change in the pulse width modulation isdependent on the adjustment in the actual position which is to bebrought about by the adjusting intervention.
 6. The method as claimed inclaim 4, wherein adjustments of the actual position which exceed alimiting value brought about in a controlled fashion, and in that afterthis control intervention the adjustment is continued to the desiredposition.
 7. The method as claimed in 1, wherein said method is used foradjusting a camshaft phase adjuster of an internal combustion engine,and wherein the actual position of the camshaft phase adjuster takesplace by sampling the position of the camshaft, at least one measurementbeing made per revolution of the camshaft.
 8. The method as claimed inclaim 1, wherein said method is used for adjusting a camshaft phaseadjuster of an internal combustion engine, and wherein the retainingpulse duty factor is obtained from a basic characteristic diagram whichcovers operating parameters of the internal combustion engine, andwherein the correction of the retaining pulse duty factor is obtainedfrom an adaptation characteristic diagram which covers the constantdeviation between the desired position and actual position.
 9. A methodfor adjusting an actuator which can move between two end positions,which is displaced into one end position and can be moved into the otherend position by activating an adjusting unit, the method comprising:sensing the actual position of the actuator; adjusting the actuator to adesired position by means of pulse-width-modulated actuation of theadjusting unit; holding the actuator in the desired position byactuation using a retaining pulse duty factor and, only if a minimumadjustment of the actuator is necessary, an adjusting intervention usingthe retaining pulse duty factor of a deviating pulse width modulation;and when there is a drift in the actual position between the repeatedadjusting interventions, determining a drift absolute value from themaximum error between the actual position and desired position,determining a drift time, and obtaining the correction of the retainingpulse duty factor from the drift absolute value and the drift time. 10.The method as claimed in claim 9, wherein a defect in the actuator isdetected if the correction of the retaining pulse duty factor isnecessary beyond a specific pulse width modulation.
 11. The method asclaimed in claim 9, wherein a defect in the actuator is detected if thecorrection of the retaining pulse duty factor is necessary beyond apredetermined time period.
 12. The method as claimed in claim 9, whereina control intervention brings about only a certain maximum change in thepulse width modulation.
 13. The method as claimed in claim 12, whereinthe maximum change in the pulse width modulation is dependent on theadjustment in the actual position which is to be brought about by theadjusting intervention.
 14. The method as claimed in claim 12, whereinadjustments of the actual position which exceed a limiting value broughtabout in a controlled fashion, and in that after this controlintervention the adjustment is continued to the desired position. 15.The method as claimed in 9, wherein said method is used for adjusting acamshaft phase adjuster of an internal combustion engine, and whereinthe actual position of the camshaft phase adjuster takes place bysampling the position of the camshaft, at least one measurement beingmade per revolution of the camshaft.
 16. The method as claimed in claim9, wherein said method is used for adjusting a camshaft phase adjusterof an internal combustion engine, and wherein the retaining pulse dutyfactor is obtained from a basic characteristic diagram which coversoperating parameters of the internal combustion engine, and wherein thecorrection of the retaining pulse duty factor is obtained from anadaptation characteristic diagram which covers the constant deviationbetween the desired position and actual position.
 17. A method foradjusting an actuator which is movable between two end positions, themethod comprising: sensing the position of the actuator; adjusting theactuator to a desired position using an adjusting unit that is actuatedwith pulse width modulation; holding the actuator in the desiredposition by actuation using a retaining pulse duty factor and, only if aminimum adjustment of the actuator is necessary, an adjustingintervention using the retaining pulse duty factor of a deviating pulsewidth modulation; and correcting the retaining pulse duty factor if,despite repeated adjusting intervention, a deviation between the desiredposition and actual position is continuously measured.
 18. The method asclaimed in claim 17, wherein a defect in the actuator is detected if thecorrection of the retaining pulse duty factor is necessary beyond aspecific pulse width modulation, wherein a control intervention bringsabout only a certain maximum change in the pulse width modulation, andwherein the maximum change in the pulse width modulation is dependent onthe adjustment in the actual position which is to be brought about bythe adjusting intervention.